1. Field of the Invention
The present invention is in the field of multiple disk refiner assemblies utilizing axially flexible membranes for supporting the confronting refining disks and being provided with undercut portions which significantly improve the flexibility of axial movement capability in the refiner disks.
2. Description of the Prior Art
After paper stock has been treated in beaters, digesters or other pulping machines, it is usually refined by passing it between grinding or refining surfaces which break up the fibrous materials and serve to create further separation and physical modification of the fibers.
A typical pulp refiner is disclosed in Thomas U.S. Pat. No. 3,371,873. The type of refiner disclosed therein includes a rotating disk which has annular refining surfaces on one or both sides. The disk refining surfaces are in confronting relation with non-rotating annular grinding surfaces and provide a refining zone therebetween in which the pulp is worked. The rotating disk and the refining surfaces are made of inflexible material such as cast iron or a hard stainless steel. The non-rotating grinding surfaces are made of similar materials and are rigidly mounted so as to resist the torque created by the rapidly rotating disk and the pressure on the pulp material passing through the refining zone gap. Axial adjustment of the refining zone gaps is effected by axial shifting of the shaft on which the disk is mounted.
Rigid disk refiners of this type must be manufactured and assembled to close tolerances in order to set the refining zone gap width correctly. Because the loads supplied to the rigid disk are large during the refining process, a large and extremely rugged design is necessary so that the refining surface relationships do not change under load. This results in the rigid disk refiners being very costly due to the necessarily close tolerance machining, the need for large quantities of high-strength disk material, the bulky overall structure, the restrictive machine capacity, and the excessive assembly time requirements.
Substantial improvements in pulp refiners have recently been accomplished with the development of a multiple disk refiner which is usually designed to operate at a low intensity. In copending Matthew and Kirchner U.S. Pat. No. 4,531,681 entitled "Flexible Disk Refiner and Method" issued July 30, 1985 and assigned to the same assignee as the present application, there is disclosed a refining apparatus which includes a plurality of radially extending, relatively rotatable and axially confronting refining surfaces between which the suspension passes while being refined during relative rotation of the surfaces. Means are provided for effecting flow of the material radially between and across the surfaces. The supporting means employed in that application consists of resiliently flexible supporting means which permit adjustment of the relatively rotating refining surfaces axially relative to each other depending on the operating pressure so that optimum material working results from the refining surfaces.
In the specific embodiment disclosed in the aforementioned application, there is provided a pulp refiner with ring-shaped refining surface plates of limited radial width which are mounted on interleaved margins of axially resilient flexible or deflectable disk elements or membranes. Disk margins spaced from the interleaved margins on one set of the disk elements are secured to a rotor while the margins on another set of disks are secured nonrotatably or counter-rotatably. The refining surface plates are made of a suitably hard, substantially rigid material. The disk elements on the other hand are made of axially resilient flexible material which strongly resists deformation in the circumferential direction. Because of the manner in which the axially flexible disk elements are supported, there is an automatic axial self-alignment of the refining surfaces during the pulp-refining process for attaining optimum refining action by the relatively rotating refining surfaces.
The multiple disk refiner represents a substantial improvement in the art of refining. It has been shown that with the use of a low intensity, multiple disk refiner pulp characteristics can be improved considerably over those obtained by using conventional refining techniques. Originally, such refiners were built using flexible diaphragms to restrain the refining disks and to provide the torsional rigidity and strength required to transmit rotational forces into the refining surfaces. The resiliency of the diaphragms permits sufficient axial motion of the refiner disks such as required as each surface moves into close proximity to its adjacent neighbors as the refiner is loaded to its operational position.
In the usual multi-disk refiner, a fiberglass composite membrane is used to achieve axial flexibility, the refiner disks being attached to the membranes. To maintain a minimal force gradient and thus uniform refining properties across the disk pairs, a low axial spring constant characteristic of the disk is required. The axial flexibility is a function of the properties of the material and the geometry.